An important step in the development of an absorbent article, for product improvement, is performing appropriate evaluation of the distribution of state quantities, such as the distribution of moisture, distribution of humidity or distribution of temperature, on the absorption surface within specific regions of the absorbent article, when a liquid has been absorbed.
For example, experience in regard to the feeling of wetness in an absorbent article indicates that the wearer does not feel wetness in proportion to the total amount of liquid absorbed by the absorbent article, but rather tends to feel wetness in proportion to the amount of moisture remaining on the very top of the absorption surface of the absorbent article. This is because liquids absorbed inside the absorbent article, and especially in the absorber, do not readily seep out of the absorbent article even when pressure, such as body pressure, is applied. It is therefore important to appropriately evaluate state quantity distributions in specific regions of the absorbent article.
However, when the moisture content on the absorption surface of an absorbent article, for example, is to be evaluated, the method currently employed in the technical field involves, as shown in FIG. 8, dropping a liquid, such as artificial excreted fluid onto an absorbent article 24, placing an absorbent sheet 25, such as filter paper or tissue on the horizontally positioned absorbent article, applying a constant load by a weight 26 set on the absorbent sheet, and then measuring the amount of liquid that has migrated into the absorbent sheet after a prescribed period of time (PTLs 1-3).
In this evaluation method, however: (1) irregularities in the absorption surface of the absorbent article create sections that contact the absorbent sheet and sections that do not contact it, while the applied pressure also differs for different sections of the absorption surface. The inability to measure potentially skin-contacting regions in a comprehensive and consistent manner has therefore been a problem. Additional problems encountered with this evaluation method include that: (2) migration of liquid depends on the absorption power of the absorbent sheet, and the large variation in the absorbancy of different absorbent sheets leads to significant measurement error, and (3) the low amount of migration of liquid into the absorbent sheet also leads to significant measurement error. Furthermore, this evaluation method does not allow measurement of state quantity distributions in specific regions of the absorbent article.
Measurement of a state quantity in a specific region of an absorbent article can be accomplished using a sensor, such as a moisture sensor, humidity sensor, temperature sensor, heat flow rate sensor or wind speed sensor.
However, since the absorption surface of an absorbent article will generally have irregularities, the distribution of a state quantity on the absorption surface of the absorbent article cannot be easily measured under consistent conditions, such as under constant pressure.
A moisture sensor, for example, may also be used to measure the moisture content on the absorption surface of an absorbent article, and known moisture sensors include electrical capacitance, infrared-absorbing, microwave and electrical resistance moisture sensors. Of these, electrical capacitance-type moisture sensors are considered most suitable for measurement of moisture contents in specific regions of absorbent articles.